This invention relates to photohardenable coating compositions, and more particularly to improved films and coatings having high resolution. Still more particularly, it relates to the addition of certain tetracyanoethane compounds to photohardenable coating compositions to obtain high resolution.
2. Description of the Prior Art
Processes for imagewise photohardening photohardenable films and coatings wherein unsaturated compounds are polymerized or suitable polymers are cross-linked by free radicals generated photochemically are well-known in the art. In these processes radiation is normally absorbed to provide the initiating species. The amount of radiation available for the production of initiating species will, therefore, decrease progressively from the surface downwards. In the deeper layers of the composition, hardening will be correspondingly less rapid. At the same time, radiation will be scattered into nonilluminated areas by the film and by reflection from the substrate thus initiating hardening in those areas. Further, active initiating species can diffuse from illuminated to nonilluminated areas. These phenomena tend to limit resolution.
Conventional polymerization inhibitors can improve resolution somewhat. This is because inhibitors destroy the initiating radicals until sufficient intiating radicals have been generated to completely consume the inhibitor, whereupon hardening will then proceed. Since the radiation entering nonilluminated areas by scattering, etc., generally has a lesser intensity than in the illuminated areas, the effect of the inhibition is greatest in such areas. The concentration of inhibitor necessary to prevent photohardening caused by scattered radiation is so high, however, that undesirable retardation of photohardening in the exposed areas of the film occurs.
High resolution has been obtained heretofore in photopolymerizable coating compositions by the use of nitroso dimers. In U.S. application Ser. No. 697,947, filed June 21, 1976, Nacci and Pazos claim photohardenable films consisting essentially of:
A. a substantially solid organic composition comprising an addition-polymerizable, nongaseus ethylenically unsaturated compound capable of forming a high polymer by free-radical initiated chain-propagating addition polymerization, and a macromolecular organic polymeric binder,
B. 0.04-3% BY WEIGHT, BASED ON THE TOTAL FILM MATERIAL, OF SOLVENT-SOLUBLE NITROSO DIMER WHICH IS A NONINHIBITOR OF FREE-RADICAL POLYMERIZATION BUT DISSOCIATES TO NITROSO MONOMER WHICH IS AN INHIBITOR OF FREE-RADICAL POLYMERIZATION, CONTAINS A DINITROSO GROUP, AND IS CHARACTERIZED BY A DISSOCIATION CONSTANT OF 10.sup.-2 -10.sup.-10 IN SOLUTION AT 25.degree. C and a rate of dissociation having a half-life of at least 0.5 minute in solution at 25.degree. C, and
c. 1.0-10% by weight, based on the total film material, of an organic, radiation-sensitive free-radical generating system activatable by actinic radiation which initiates polymerization of the unsaturated compound and does not dissociate the nitroso dimer to nitroso monomer, with the proviso that the weight ratio of nitroso dimer to free-radical generating system is no greater than 2:1, said free-radical generating system being proportioned to give said film at its particular thickness a mean optical density of 0.02-1.25 to radiation in the spectral region in which the free-radical generating system absorbs radiation.